1. Field of the Invention
The invention relates to microactuators that are capable of positioning objects on extremely fine-length scales. More specifically, the invention relates to instruments utilizing microactuation for positioning objects with subnanometer precision. The invention is useful in fields ranging from surface measurement applications such as scanning probe microscopy to surface modification applications such as single point diamond turning.
2. Discussion of the Related Art
As miniaturization of components in a wide range of devices hastens, the need for positioning objects such as probes and tools precisely and quickly on smaller and smaller length scales correspondingly increases. This trend is very important in surface modification instruments, because smaller and smaller devices are being built, requiring high precision, stability, and speed. For example, some diamond turning machines must be capable of machining parts to within subnanometer tolerances. This trend is equally important in measuring instruments, such as profilometers and scanning probe microscopes (SPMs) including atomic force microscopes (AFMs) and magnetic force microscopes (MFMs), that measure topography and other and subsurface surface characteristics of samples. In fact, the need for precision in measuring instruments is even more acute, given the fact that the instruments must be capable of rapidly resolving dimensions smaller than those being fabricated in order to assure manufacturing quality and accurate diagnoses of manufacturing problems. In short, the measuring instrument must have higher resolution and precision than that of the fabrication device. For the sake of convenience, the discussion that follows will focus on AFMs, it being understood that the problems addressed and solved by the invention are also experienced by other measurement instruments, a wide range of surface modification instruments, and other microactuated devices.
The typical AFM includes a probe which includes a flexible cantilever and a tip mounted on the free end of the cantilever. The probe is mounted on a scanning stage that is typically mounted on a common support structure with the sample. The scanning stage includes an XY actuator assembly and a Z actuator. The XY actuator assembly drives the probe to move in an X-Y plane for scanning. The Z actuator, which is mounted on the XY actuator assembly and which supports the probe, drives the probe to move in a Z axis extending orthogonally to the X-Y plane.
AFMs can be operated in different modes including contact mode and TappingMode. In contact mode, the cantilever is placed in contact with the sample surface, and cantilever deflection is monitored as the tip is dragged over the sample surface. In TappingMode (Tapping and TappingMode are trademarks of Veeco Instruments Inc.), the cantilever is oscillated mechanically at or near its resonant frequency so that the tip repeatedly taps the sample surface or otherwise interacts with the sample, reducing the cantilever's oscillation amplitude. U.S. patents relating to Tapping and TappingMode include U.S. Pat. Nos. 5,266,801, 5,412,980 and 5,519,212, by Elings et al.
In any operational mode, interaction between the probe and the sample induces a discernable effect on a probe operational parameter, such as cantilever deflection oscillation amplitude, phase or frequency, that is detectable by a sensor. The resultant sensor-generated signal is used as a feedback control signal for the Z actuator to maintain a designated probe operational parameter constant. In contact mode, the designated parameter may be cantilever deflection. In TappingMode, the designated parameter may be oscillation amplitude, phase or frequency. The feedback signal also provides a measurement of the surface characteristic of interest. For example, in TappingMode, the feedback signal is used to maintain the amplitude of probe oscillation constant to measure the height of the sample surface or other sample characteristics.
During AFM operation, the maximum data acquisition rate is dependent to a large extent on the bandwidth of the feedback loop for the Z actuator. For instance, a Z actuator controlled by a feedback loop having a one KHz bandwidth is capable of following 1,000 surface oscillations per second. Historically, it was commonly thought that the resonance of the Z actuator was the greatest limiting factor on the bandwidth of the feedback loop. Prior efforts at increasing Z feedback loop bandwidth therefore focused on increasing the resonant frequency of the Z actuator. However, the inventors have realized that bandwidth is also limited by induced resonant motions in other components of the AFM during feedback control of the Z actuator.
A typical AFM has a mechanical loop extending from the sample support structure, through the support structure for the scanning stage, through the XY actuator assembly of the scanning stage, and to the Z actuator. The bandwidth limitation of the AFM is typically equal to the lowest excited natural frequency of the subcomponents of the AFM's mechanical loop. The inventors have therefore identified the need to eliminate induced motion in the supporting structure for a microactuator. However, heretofore, this need has not been previously satisfied.